This case involves primarily the constitutionality of the Act of June 28, 1938, 52 Stat. 1215, insofar as it authorizes the construction of the Denison Reservoir on Red River in Texas and Oklahoma.
1
[313
U.S. 508, 511]
The bill in equity was filed by the State of Oklahoma seeking to enjoin the construction of any dam across Red River within the domain of Oklahoma which would impound the waters of the Red River (or its tributary, Washita River) so as to inundate and destroy any of the lands, highways or bridges belonging to or under the jurisdiction and control of the state or which would obliterate or interfere with its boundaries. The bill also seeks to restrain the institution or conduct in any court in Oklahoma of proceedings to condemn lands for the purpose of the dam or reservoir.
2

The bill alleges that Oklahoma will be injured in the following manner by construction of the project: The greater part of the dam will rest on Oklahoma soil and will form a reservoir inundating about 150,000 acres of land, of which 100,000 acres are located in Oklahoma. Of those acres about 3,800 are owned by the state. The United States will acquire title to the inundated land. The land owned by the state is used for school purposes, for a prison farm, for highways, rights of way, and bridges. The basin to be inundated is inhabited by about 8,000 Oklahoma citizens. Much of the land is rich soil in a high state of
[313
U.S. 508, 512]
cultivation. Much of it has large potential oil reserves. On some of it there are large producing oil wells and on other parts there are drilling operations and exploration for oil and gas. At least 15,000 acres will be highly productive oil lands and at least 50,000 acres are underlaid with oil and gas. There are thirty-nine school districts and townships in the four counties in which the affected area is located. Those governmental units are largely supported by ad valorem taxes. The taking of the 100,000 acres wil decrease the taxable property in each of the counties and take virtually all of the taxable property in many of the t wnships and school districts. Each of these governmental units has a large bonded indebtedness payable from an annual levy of taxes. Inundation of the land will deprive those units of much of the tax revenue, so that many will be practically destroyed and the remainder seriously hampered. Since the state derives much of its revenue from a gross production tax on oil and gas, it will suffer great losses in tax revenues from the inundation of the oil and gas lands. The 'annual wealth production' to the citizens of Oklahoma from the lands in the reservoir basin is about $1,500,000. Aside from such losses and losses from oil revenues and personal property taxation, the net taxable loss to the counties, townships and school districts will be about $40,000 annually.

It is also alleged that the construction of the dam will be a 'direct invasion and destruction' of the sovereign and proprietary rights of Oklahoma in that: the boundary of Oklahoma will be obliterated for approximately 40 miles (see Oklahoma v. Texas,
260
U.S. 606
, 43 S.Ct. 221); there will be a 'forcible reduction of the area of plaintiff as one of the United States'; lands owned by it will be taken; its highways and bridges will be destroyed causing an interruption in communication between various parts of the state; the waters to be impounded belong to Oklahoma but will be taken from it without payment of just compensation;
[313
U.S. 508, 513]
those waters will be diverted from Oklahoma and will be run through turbines located in Texas for the generation of power for sale principally in Texas; the removal of citizens from the 100,000 acres of land will create a 'serious social and economic problem', the burden of which will fall on Oklahoma for which no compensation is afforded.

The bill incorporates H.Doc.No.541, 75th Cong., 3d Sess. (hereinafter called the Report) which contains the War Department's survey and recommendations on the Denison Reservoir and which served as the broad definition of the project which was authorized by the Act of June 28, 1938. The bill alleges that under the statutory scheme flood control and power purposes are 'inextricably and inserverably involved'. It alleges that, as described in the Report, the first 110 feet of the dam are to be used 'solely and exclusively for the development of waterpower', while 40 feet 'superimposed' on the power reservoir are to be used 'solely and exclusively' for flood control. That is to say, from elevation 510 feet ( sea level) to 590 feet there is to be a dead storage pool for waterpower head, from 595 feet to 620 feet there is to be a water power reservoir, and from 620 feet to 660 feet there is to be a flood control reservoir. It is alleged that those purposes are 'functionally separate and neither is the incidental or necessary result of the other'; that the same part of the reservoir will not and cannot be used for both flood control and waterpower purposes; and that the power portion of the dam is created at the expense of its utilization for flood control. The bill further alleges that as a result of the modification of the statutory plan set forth in the Report the dam is being constructed so as to provide dead storage for water head from 510 feet to 567 feet, a power pool reservoir from 587 feet to 617 feet and a flood control reservoir from 617 feet to 640 feet. It is alleged that by reason of that modification the reservoir
[313
U.S. 508, 514]
will inundate 3,080,000 acre feet for power and 2,745,000 acre feet for flood control as contrasted to 3,400,000 acre feet for power and 5,900,000 acre feet for flood control under the original plan;3 and that, as a result, the statutory
[313
U.S. 508, 515]
scheme has been charged from one preponderantly for flood control to one preponderantly for water power. It is also alleged that no part of the Red River in Oklahoma is navigable.

The bill alleges that the Act under which appellees are proceeding is unconstitutional in that it violates the Tenth Amendment, that it is not within the powers of Congress conferred by Art. I, Sec. 8 of the Federal Constitution, and that since appellees are acting under a void and unconstitutional statute they should be enjoined. By an amendment to its bill, the state of Oklahoma also challenges the constitutionality of 4 of the Act of October 17, 1940, 54 Stat. 1200,4 Pub. No. 868, c. 895, 76th Cong., 3d Sess. The amended bill alleges that the project 'does not in any way protect or improve the navigable portions of the lower reaches of Red river or of the Mississippi river either by enriching the low water flow ... as the incidental result of the operation of said flood control and hydroelectric power project, except in the intangible, indirect, inconsequential and unsubstantial way' set forth in the Report; and that such inconsequential and intangible benefits to navigation as may result will flow from the flood control, not the power feature, of the project.
[313
U.S. 508, 516]
By motions to dismiss the appellees asserted, inter alia, that the Acts of Congress so challenged were constitutional and valid. The case was heard by a three judge court (Act of August 24, 1937, c. 754, 3, 50 Stat. 751, 28 U.S.C. 380a, 28 U.S.C.A. 380a) which sustained the Act authorizing the project. D.C., 37 F.Supp. 93. From a judgme t dismissing the complaint and denying the injunction, a direct appeal was taken to this Court.

We are of the view that the Denison Dam and Reservoir project is a valid exercise of the commerce power by Congress.

This project is a part of a rather recent chapter in the long history of flood control on the Mississippi River.
5
The federal government had concerned itself with the problems of navigation and flood control on that river long before6 the establishment of the Mississippi River Commission, 21 Stat. 37, 33 U.S.C.A. 641 et seq., in 1879. Earlier efforts towards a more comprehensive flood control program on a national scale7 were accelerated by the disastrous Mis-
[313
U.S. 508, 517]
sissippi flood in 1927. The agitation and concern over that disaster8 led to the enactment of the Flood Control Act of May 15, 1928, 45 Stat. 534, 10 of which provided that the Secretary of War should submit to Congress 'at the earliest practicable date projects for flood control on all tributary streams of the Mississippi River system subject to destructive floods which projects shall include: The Red River and tributaries ....' 33 U.S.C.A. 702j. That section of the Act also required a report on the effect on flood control of the lower Mississippi to be attained through the use of a reservoir system, the 'benefits that will accrue to navigation and agriculture' from the prevention of siltage and erosion, the 'prospective income from the disposal of reservoired waters', and 'inquiry as to the return flow of waters placed in the soils from reservoirs, and as to their stabilizing effect on stream flow as a means of preventing erosion, siltage, and improving navigation.' Pursuant to that authorization and direction a report (H.Doc.No.378, 74th Cong., 2d Sess.) was submitted on December 2, 1935, dealing at great length with the problems of the Red River and its tributaries and their relationship with the Mississippi.

On June 22, 1936, there was enacted9 the Flood Control Act of 1936, 49 Stat. 1570, 33 U.S.C.A. 701a et seq. Section 1 of the Act set
[313
U.S. 508, 518]
forth a broad Congressional policy, stating, inter alia, that 'the Federal Government should improve or participate in the improvement of navigable waters or their tributaries, including watersheds thereof, for flood- control purposes if the benefits to whomsoever they may accrue are in excess of the estimated costs, and if the lives and social security of people are otherwise adversely affected' and that 'destructive floods upon the rivers of the United Sta es, upsetting orderly processes and causing loss of life and property, including the erosion of lands, and impairing and obstructing navigation, highways, railroads, and other channels of commerce between the States, constitute a menace to national welfare.' That Act authorized the construction of various flood control projects. By 7 of that Act the Secretary of War was authorized and directed to continue the investigation of other projects, including the Denison Reservoir, where 'opportunities appear to exist for useful flood-control operations with economical development of hydroelectric power whenever sufficient markets to absorb such power become available.'

Following the disastrous Ohio River flood in January, 1937, the House Committee on Flood Control requested10 the Chief of Engineers to submit 'comprehensive plans for protective works against floods in the Ohio Valley' and plans 'to further insure protection in the Mississippi Valley'. He submitted a report pursuant to that direction and recommended the construction of 45 flood-control reservoirs on the tributaries of the Ohio and 24 on other tributaries of the Mississippi, including the Red River.
11
As to the proposed Denison Reservoir he stated that it 'would remove the threat of the coincidence of a
[313
U.S. 508, 519]
large flood from the Red with a flood in the Mississippi, and would also afford highly desirable protection to the fertile bottom lands in the lower Red River Valley. Besides its flood-control benefits, it has valuable potentiality for power purposes.'
12
And he added: 'On the Red River ... investigations indicate that a flood far exceeding any of record is distinctly possible. The Denison Reservoir would prevent such a flood from reaching disastrous proportions in the valley below it.'
13

On March 12, 1938, the Acting Secretary of War transmitted to Congress a report from the Chief of Engineers, United States Army, pursuant to the direction contained in 7 of the Flood Control Act of 1936. That Report, being the one here involved, (H.Doc.No.541, 75th Cong., 3d Sess.) recommended the construction of a dam near Denison, Texas, for the combined purpose of flood control and development of hydroelectric power. After hearings14 Congress passed the Flood Control Act of 1938, here challenged, which authorized,15 inter alia, the Denison project on the basis of the Report and at an estimated cost of $54,000,000. This was followed by appropriations for the construction work16 and by the
[313
U.S. 508, 520]
Act of October 17, 1940, also challenged by appellant, declaring the Denison Reservoir to be 'for the purpose of improving navigation, regulating the flow of the Red River, controlling floods, and for other beneficial uses.'
17
Thus, while the Report spoke of the dam as a 'dual purpose' project, Congress did not so limit it but authorized it for multiple purposes.

From this history it is plain that this project, which is part of a comprehensive flood control plan, is designed to control the watershed of one of the principal tributaries of the Mississippi in alleviation of floods in the lower Red River and Mississippi valleys. The Red River, sixth in length among rivers in the United States, has one of the largest watersheds in the country, draining an area about 50 per cent larger than New England-an area of 91,430 square miles, of which 38,291 square miles are above the dam site.
18
It rises near the east edge of New Mexico, flows easterly about 850 miles across the Texas Panhandle and between the states of Oklahoma and Texas to Fulton, Arkansas. From there it flows south and southeast some 460 miles and enters the Mississippi at Red River Landing. The site of the Denison dam is 228 miles up the river from Fulton. The contribution which the Red River makes to disastrous floods in its basin and in the lower Mississippi has long been recognized. Huge crop damage, the loss of buildings, bridges and livestock, pollution of fertile fields, the erosion of rich farm lands, bank cavings, interruption of navigation, injury of port facilities, the creation of sand bars in the channels, interruption or stoppage of interstate transportation by rail, truck and motorcar, disease, pestilence and death, relief of the homeless and destitute-all these are now familiar costs of the floods on the Missis-
[313
U.S. 508, 521]
sippi.
19
And the history of the Red River valley shows that it has long been plagued by such disasters and burdened by their costs.
20

Floods pay no respect to state lines.
21
Their effective control in the Mississippi valley has become increasingly a subject of national concern22 in recognition of the fact
[313
U.S. 508, 522]
that single states are impotent to cope with them effectively. The methods of dealing with them have elicited a contrariety of views.
23

The idea of reservoir control on the tributaries of the Mississippi is not new. The Ellet report24 to the War Department in 1852 urged the making of surveys for the installation of reservoirs on the Red River and other tributaries which would serve the 'double purpose' of 'keeping back the floods' and relieving 'summer navigation from obstruction, by allowing the surplus so retained, to pass down in the season of low water.'
25
The emergence in recent years of comprehensive plans for reservoirs in the Mississippi river basin26 marks the development of an integrated system designed not only to alleviate ultimately flood conditions on the Mississippi itself but also to avoid or reduce local flood disasters. A part of the local benefits of flood control is frequently
[313
U.S. 508, 523]
protection of navigation in the tributary itself. That is present here to a degree. It is true that 'no part of the (Red) river within Oklahoma is navigable.' Oklahoma v. Texas,
258
U.S. 574, 591
, 42 S.Ct. 406, 413. Though appellant alleged that the stream is not now a navigable river of the United States, it has heretofore been authoritatively determined that in years past 'the usual head of navigation' was Lanesport, Arkansas, near the Oklahoma boundary. Id., 258 U.S. page 589, 42 S.Ct. page 412. At the present time commerce on the Red River is limited to the section below Alexandria, Louisiana, 122 miles above its mouth.
27
The fact that portions of a river are no longer used for commerce does not dilute the power of Congress over them. Economy Light & Power Co. v. United States,
256
U.S. 113, 123
, 41 S.Ct. 409, 412; United States v. Appalachian Power Co.,
311
U.S. 377, 409
, 410 S., 61 S.Ct. 291, 300. And it is clear that Congress may exercise its control over the non-navigable stretches of a river in order to preserve or promote commerce on the navigable portions. United States v. Rio Grande Dam & Irrigation Co.,
174
U.S. 690, 703
, 706 S., 708, 19 S.Ct. 770, 775-777; United States v. Utah,
283
U.S. 64, 90
, 51 S.Ct. 438, 446. It is obvious that at least incidentally Congress has done precisely that in this case. Congress was not unmindful of the effect of this project on the navigable capacity of the river. In authorizing it, Congress exercised all the power it possessed to control navigable waters. The Acts in question contain a declaration that one of their purposes is to improve navigation. And the Report clearly shows that the Denison Reservoir will have at least an incidental effect in protecting or improving the navigability of portions of the Red River. The District Engineer reported that 'Inasmuch as any new navigation system for the Red River would require flow regulation to furnish a dependable navigable improvement, the Denison
[313
U.S. 508, 524]
Reservoir would be of considerable benefit.'
28
In his view it would decrease bank caving and silt carriage, substitute 'moderately high stages of long durations for high-flood stages of short duration', 'furnish more dependable naviga le stages especially in the upper portions of the navigation pools',29 and have a 'favorable effect on open-channel navigation by reducing flood stages and increasing low-water flows.'
30
The Division Engineer expressed the view that a 'dependable low-water flow of 2,200 to 3,000 cubic feet per second which would result from construction and operation of the power project at Denison would be of distinct benefit to the small commerce now developed upon those reaches of the lower Red River which are included in approved navigation projects, and might have a material bearing upon future studies of the Red River with a view to its further improvement. In the present state of knowledge upon this point, it is necessary to classify these benefits among the intangibles. But there is no doubt that a dependable low water supply would simplify, perhaps materially, such future development of the river as may be undertaken.'
31
Thus the effect on the river is tangible, though the value may be uncertain32 since it de-
[313
U.S. 508, 525]
pends in part on future action of Congress. But that is not our concern.

We would, however, be less than frank if we failed to recognize this project as part of a comprehensive flood control program for the Mississippi itself. But there is no constitutional reason why Congress or the courts should be blind to the engineering prospects of protecting the nation's arteries of commerce through control of the watersheds. There is no constitutional reason why Congress cannot under the commerce power treat the watersheds as a key to flood control on navigable streams and their tributaries. Nor is there a constitutional necessity for viewing each reservoir project in isolation from a comprehensive plan covering the entire basin of a particular river. We need no survey to know that the Mississippi is a navigable river. We need no survey to know that the tributaries are generous contributors to the floods of the Mississippi. And it is common knowledge that Mississippi floods have paralyzed commerce 33 in the affected areas and have impaired navigation itself. We have recently recognized that 'Flood protection, watershed development, recovery of the cost of improvements through utilization of power are ... parts of commerce control.' United States v. Appalachian Power Co., supra, 311 U.S. page 426, 61 S.Ct. page 308. And we now add that the power of flood control extends to the tributaries of navigable streams. For just as control over the non-navigable parts of a river may be essential or desirable in the interests of the navigable portions,
[313
U.S. 508, 526]
so may the key to flood control on a navigable stream be found in whole or in part in flood control on its tributaries. As repeatedly recognized by this Court from McCulloch v. Maryland, 4 Wheat. 316, to United States v. Darby,
312
U.S. 100
, 61 S.Ct. 451, 85 L.Ed. --, 132 A.L.R. 1430, the exercise of the granted power of Congress to regulate interstate commerce may be aided by appropriate and needful control of activities and agencies which, though intrastate, affect that commerce.

It is, of course, true that the extent to which this project will alleviate flood conditions in the lower Mississippi is somewhat conjectural. The District Engineer estimated that the Denison project would cause a reduction of 35,000 cubic feet per second in the lower Mississippi in case the May, 1908 flood were repeated; 8,000 cubic feet per second, in case of the May, 1935 flood; and 100,000 cubic feet per second, in case of the estimated maximum probable flood.
34
But the Division Engineer pointed out that 'the magnitude of the effect would depend upon the size and origin of the concurrent flood in Red River, and upon the basis of reservoir operation.'
35
In his view, a reduction in flow of 35,000 cubic feet per second in case of such a flood as 1908 'if long enough sustained, would imply a reduction in stage averaging 1.3 feet between Alexandria and Moncla, and a reduction of 0.15 foot in the flow lines of the Atchafalaya Basin and the main river below Old River, provided they were at peak stage. At lower stages the effect would be greater, but less necessary.'
36
This matter was again reviewed in the Definite Project and the following observations were made:37 'Floods in the Missis-
[313
U.S. 508, 527]
sippi River usually occur in the spring as a result of flood flows out of the Ohio River. The coincidence of flood flows out of the Red River with the Mississippi River spring floods is rare. However, the early summer floods out of the Missouri River occasionally coincide in the Mississippi River with the summer floods out of the Red River. The control provided by the proposed Denison Dam and Reservoir on the Red River summer floods has been estimated to produce a reduction of approximately 0.6 foot at the mouth of Old River on the Mississippi. This reduction, while not substantial with respect to Mississippi flood stages is important when flood crests seriously tax the Mississippi levee system.'

Such matters raise not constitutional issues but questions of policy. They relate to the wisdom, need, and effectiveness of a particular project. They are therefore questions for the Congress not the courts. For us to inquire whether this reservoir will effect a substantial reduction in the lower Mississippi floods would be to exercise a legislative judgment based on a complexity of engineering data. It is for Congress alone to decide whether a particular project, by itself or as part of a more comprehensive scheme, will have such a beneficial effect on the arteries of interstate commerce as to warrant it. That determination is legislative in character. Cf. United States v. Appalachian Power Co., supra, 311 U.S. page 424, 61 S. Ct. page 307. The nature of the judgment involved is reemphasized if this project is viewed not in isolation but as part of a comprehensive, integrated reservoir system in the Mississippi River basin. A War Department survey in 1935 reveals promising engineering prospects in a system of 157 reservoirs38 throughout the tributaries of the Mississippi. To say that no one of those projects could be constitutionally authorized because its separate effect on floods in
[313
U.S. 508, 528]
the Mississippi would be too conjectural would be to deny the actual or potential aggregate benefits of the integrated system as a whole. That reveals the necessity from the constitutional viewpoint of leaving to Congress the decision as to what watersheds should be controlled (and what methods should be employed) in order to protect the various arteries of interstate commerce from the disasters of floods.

Nor is it for us to determine whether the resulting benefits to commerce as a result of this particular exercise by Congress of the commerce power outweigh the costs of the undertaking. Arizona v. California,
283
U.S. 423, 456
, 457 S., 51 S.Ct. 522, 526, 527; Ashwander v. Tennessee Valley Authority,
297
U.S. 288, 329
, 330 S., 56 S.Ct. 466, 474, 475. Nor may we inquire into the motives of members of Congress who voted for this project in an endeavor to ascertain whether their concern over the great national loss from soil erosion, the enormous crop damages, the destruction of homes, the loss of life and other like ravages of floods overshadowed in their minds the desirability of protecting the Mississippi and other arteries of commerce. Arizona v. California, supra, 283 U.S. page 455, 51 S.Ct. page 526, and cases cited. It is sufficient for us that Congress has exercised its commerce power, though other purposes will also be served. Id., 283 U.S. page 456, 51 S.Ct. page 526

But Oklahoma points out that the Denison Reservoir is a multiple purpose project,39 combining functionally and physically separate and unrelated purposes. It says that only the top 40 feet of the dam is set apart for flood control and that the lower portions of the dam are designed for the power project and are neither useful or necessary for flood control. It points out from the Report40 that a reservoir for flood control only would have
[313
U.S. 508, 529]
a maximum height of 165 feet while a reservoir for flood control and power development would require a maximum height of 185 feet. It therefore earnestly contends that the additional 20 feet in height of the dam requires a very much greater acreage of appellant's domain than would a project for flood control only. And it insists that Congress is without authority to authorize a taking of Oklahoma's domain for the construction of the water power feature of the project.

There are several answers to these contentions. We are not concerned here with the question as to the authority of the federal government to establish on a non-navigable stream a power project which has no relation to, or is not a part of, a flood control project. While this reservoir is a multiple purpose project, it is basically one for flood control. There is no indication that but for flood control it would have been projected. It originated as part of a comprehensive program for flood control. And the recommendation in the Report that a dual purpose dam be constructed was based 'on the assumption that the f ood-control project is to be built in any event.'
41
See United States v. Chandler-Dunbar Co.,
229
U.S. 53, 73
, 33 S.Ct. 667, 676. Furthermore, it is plain from the Report that the construction of the project so as to accommodate power will increase or augment some of the flood control benefits, including river flow, which would accrue were the dam to be erected for flood control only. Thus, the District Engineer stated: 'If it were con-
[313
U.S. 508, 530]
structed solely for flood control it would have beneficial effects in reducing floods, decreasing bank caving and silt carriage, and in substituting moderately high stages of long durations for high-flood stages of short duration. If the Denison Reservoir were constructed for the dual purposes of flood control and power development, these beneficent effects would be augmented by those resulting from the regulated power discharge which would increase low-water flows and furnish more dependable navigable stages especially in the upper portions of the navigation pools.'